This invention relates to a processing method for producing high resolution radar imagery and accurate dimensional measurements using synthetic or inverse synthetic aperature radar. The data is from an airborne radar using a "stretch" format, that is, a long linear FM waveform is used for each individual radar pulse, and the returned waveforms are then mixed on reception with a reference chirp.
In many radar application it is necessary to form two-dimensional images of targets such as ground vehicles, aircraft, ships, and so forth. Resolution in one dimension is provided by range resolution, and resolution in the other dimension is provided by Doppler resolution. The principles are widely applicable and widely applied. Synthetic aperture radar forms two-dimensional images in range and cross range on this basis by utilizing the motion of the platform for Doppler resolution. Inverse synthetic aperture radar is accomplishing the same objective by utilizing the motion of the target to be imaged. In other applications the same principle of Doppler resolution is used, even though no specific name has been given to the process. The very same principles also if, instead of forming images, the processor uses range and Doppler resolution to resolve specific scatterers on the target, and then measures the separation of these scatterers in order to obtain target dimensions.
A serious problem appears with Doppler resolution for man-made targets. Underlying the principles of Doppler resolution is the assumption that a target can be modeled by a set of fixed point scatterers. The emphasis here is placed on "point" scatterers, which term implies that the backscattering behavior is not aspect-angle dependent. In other words, the backscattering is isotropic. In this case as the processing time is increased in order to achieve better Doppler resolution, it is indeed possible to resolve scatterers which are more closely spaced, thus obtaining an image with more detail. Unfortunately, however, man-made targets do not correspond to this model, and Doppler resolution is not working as desired.
Man-made targets such as ground vehicles or ships consist of scattering centers which are extended smooth plates, long edges rods, and the like. These scattering units typically are so large compared with the radar wavelength that they have a highly lobed backscattering pattern. In low-resolution applications coherent processing will extend over relatively small changes in the aspect angle of the target, so that the radar may stay within one and the same sidelobe of the backscattering pattern. However, when the processing time is increased in order to achieve higher Doppler resolution, there will be observed over the processing time a full or even more than one sidelobe of the backscattering pattern. Doppler resolution theory shows that under these circumstances the responses of scatterers are smeared out into meaningless background interference, so that image detail is lost. It is a serious effect which drastically degrades image quality. It is equally detrimental for dimensional measurements, since most of the scatterers will not be observable at all.
As an illustration of the problem, in FIG. 1 shows a two-dimensional image of a tank generated by conventional processing methods. The positions of responses, which then were plotted by the computer as circles. The larger circles mark the positions of stronger responses, whereas the small circles indicate responses so weak that they are more likely to be background interference. A diagram of the tank is overlaid. Note that the stronger responses form a type of L, which is characteristic for conventional images of stationary tanks. Evidently, much detail is lost. It takes a trained operator to recognize that this is the image of a stationary tank, and even for such an operator it is impossible to recognize what tank it is. Conventional imaging does not work at all when the vehicles are moving, so that no sample "image" is shown there.
U.S. patents of interest include Tricoles et al U.S. Pat. No. 4,385,301, which discloses a system providing an image of the location of emitters of electromagnetic radiation behind enemy lines on a battlefield. The patented system comprises an array of antennas for receiving electromagnetic radiation from emitters, and for providing a received signal from each emitter in response to the received radiation; a receiver system coupled to each antenna of the array for measuring the phase and intensity of each received signal, and for providing separate coherent phase signals and amplitude signals that respectively indicate the measured phase and intensity; and a signal processor coupled to the receiver system for processing the coherent phase signals and amplitude signals to provide an image signal for generating an image of the emitters. U.S. Pat. No. 4,387,373 to Longuemare Jr. in column 4, lines 3-6 speaks of improving the quality of a synthetic monopulse radar image by increasing the processing rate. A high speed ambiguity function evaluation processor is taught by Tamukra in U.S. Pat. No. 4,389,373. In U.S. Pat. No. 4,209,853, Hyatt combines a high resolution narrow field-of-view array with a low resolution wide field-of-view array in a holographic processor. In U.S. Pat. No. 4,068,234, O'Meara processes imaging information with a computer programmed for inverse Fourier transform computations, and in U.S. Pat. No. 4,204,262 Fitelson et al perform optical signal processing with a direct electronic Fourier transform.